The present invention is directed to an aging process for solid electrolytes.
An anode of a solid state tantalum electrolytic capacitor is typically made from a porous pellet of sintered tantalum. A dielectric layer, such as a tantalum pentoxide dielectric layer, is formed by anodizing the pellet in an appropriate electrolyte. Typically, cathode material for solid tantalum electrolytic capacitors is manganese dioxide. Recently capacitors employing conductive polymers as the counter electrode have become commercially available.
One of the advantages of the conductive polymer device is lower equivalent series resistance (ESR). Such low ESR is required in several capacitor applications including voltage regulation for microprocessors. As microprocessors have become more complex and operating frequencies have increased, these applications have called for higher currents, and higher capacitance, while operating voltages have decreased in order to prevent overheating of the microprocessor. As current requirements increase, further reductions in the ESR of the capacitors is necessary in order to properly regulate the operating voltage. The trend toward higher currents and lower operating voltages which require low ESR capacitors is ongoing.
Another well established trend in the electronics industry, particularly in the field of computers and telecommunications, is toward miniaturization of electronic devices. Volumetric efficiency, defined as the product of the capacitance times the rated voltage of the device per unit volume, is a critical parameter for capacitors in the electronics industry, especially where board space is limited. Due to high volumetric efficiency, tantalum capacitors are components of choice in such industries. The high volumetric efficiency of tantalum capacitors is due to the large surface area of the porous tantalum anode.
Capacitance is proportional to the surface area and inversely proportional to the anodization voltage. In order to maintain high volumetric efficiency and device reliability, the anodization voltage must be selected based on the voltage rating of the capacitor. If the anodization voltage is too low the device may fail when used in applications involving higher operating voltages than the low voltage dielectric can withstand. If the anodization voltage is too high, capacitance and volumetric efficiency decrease. Typically the anodization voltage is from 2.5 to 4 times the rated voltage of the capacitor.
Cathode material is selected for solid tantalum capacitors based on its ability to isolate flaws in the dielectric. For instance, after coating the surfaces of the dielectric (e.g. a tantalum pentoxide dielectric) with the cathode material (either manganese dioxide or conductive polymer), defect sites in the dielectric are then isolated by reanodizing. Voltage is applied causing current to flow through the flaw sites in the dielectric film. As the current flows through the flaw site, the counter electrode material immediately adjacent to the flaw site is rendered nonconductive. For manganese dioxide, it is believed the mechanism is due to Joule heating at the flaw site, causing the temperature of the manganese dioxide immediately adjacent to the flaw site to increase due to conduction. As the temperature of the manganese dioxide immediately adjacent to the flaw site reaches the decomposition temperature of manganese dioxide (500-600xc2x0 C.), it is converted to manganese sesquioxide, thus isolating the flaw site. Leakage currents through the flaw sites decrease according to Ohm""s Law as the resistance of the cathode material surrounding the flaw site increases. A similar mechanism is postulated for conductive polymer counter electrodes. Other possible mechanisms to account for the healing mechanism of conductive polymer films include complete decomposition of the polymer adjacent to the flaw site, over oxidation of the polymer, and redoping of the polymer at the flaw site.
The so called xe2x80x98healingxe2x80x99 phenomenon can be demonstrated by plotting the current versus the applied voltage during the reanodization process. A typical reanodization process profile for a tantalum capacitor manufactured with a conductive polymer counter electrode is illustrated in FIG. 1. The tantalum oxide dielectric film was formed to 20 V at 60 xc2x0 C. Voltage and current increase during the initial stages of the reanodization process following Ohm""s Law. At approximately 9-11 volts the current peaks, and decreases rapidly while the voltage continues to increase to the set point voltage for the process (19 volts in this case). Beyond the voltage at which the current peaks, no obvious relationship between the applied voltage and current flow is observed due to the increasing resistance of the conductive polymer immediately adjacent to the dielectric flaw sites. The reanodization voltage must exceed the voltage corresponding to the peak current to generate sufficient heat to properly isolate the dielectric flaws. In related application U.S. Ser. No. 09/315,960 filed May 21, 1999, now U.S. Pat. No. 6,136,176, it was anticipated that the reanodization voltage would need to be 60 to 85% of the formation voltage for 6 and 10 volt rated parts (anodization voltages of 18 to 35 V). More recently, it has been found that for 4 volt rated parts (anodization voltages of 11 to 13 volts), the reanodization voltage needs to be 90 to 98% of the anodization voltage. The voltage at which the peak current flows is nearly independent of the anodization voltage (FIG. 2).
This presents a problem for manufacturing tantalum capacitors in the 2-3 volt rating range. The anodization voltage for these devices is 8 to 10 volts. The 9 to 11 volts, needed to cause healing of the flaw sites, exceeds the anodization voltage and this can lead to dielectric degradation.
It was discovered that the reanodization voltage can be raised above the formation voltage for brief periods of time and this causes healing of the flaw sites without damage to the dielectric film.
The invention is directed to a process for aging solid electrolytic capacitors using a pulse voltage technique to accelerate the healing mechanism and allow voltages in excess of the anodization voltage to be employed.
It is an object of this invention to reduce the leakage current of solid electrolytic capacitors impregnated with conductive polymer electrodes.
It is another object of the present invention to isolate flaw sites in the dielectric film of solid electrolytic capacitors impregnated with conductive polymer counter electrodes without damaging the dielectric film.
It is a further object of this invention to minimize decomposition of the conductive polymer counter electrode during the reanodization process.
It is yet another object of this invention to accelerate the reanodization process for isolating the flaw sites of solid electrolytic capacitors impregnated with conductive polymer electrodes.
It is another object of the invention to apply the aging process to capacitors with low voltage ratings, e.g. 2-4 volt, as well as higher voltage devices.
The invention is directed to a process for isolating flaw sites in the dielectric of solid electrolytic capacitor comprising alternately subjecting a conductive polymer impregnated capacitor to a high voltage and a low voltage; wherein the high voltage is between about 10 volts and 50 volts, and the low voltage is between about 0 volts and the voltage corresponding to 90% of the anodization voltage for pellets anodized at less than 20 volts, or the voltage at which the current drops to 50% of the peak voltage current for pellets anodized at voltages greater than or equal to 20 volts.